This invention relates to the measurement of the small magnetic fields produced by the body of a living organism, and, more particularly, to a biomagnetometer with a large array of pickup coils.
The biomagnetometer is a device that measures the very small magnetic fields produced by the body of a living organism. The magnetic fields, particularly those produced by electrical currents flowing in the brain and the heart, can be important indicators of the health of the body, because aberrations in the magnetic field can be associated with certain types of disfunctions either for diagnosis or early prediction. Moreover, the magnetic fields produced by the brain are an indicator of sensory, motor, or thought processes and the location at which such processes occur, and can be used to investigate the mechanisms of such processes.
Magnetic fields produced by the body are very small, because they result from very small electrical current flows. Typically, the strength of the magnetic field produced by the brain is about 0.00000001 Gauss. By comparison, the strength of the earth's magnetic field is about 0.5 Gauss, or over ten million times larger than the magnetic field of the brain.
The biomagnetometer must therefore include a very sensitive sensor of magnetic fields and sensor channels to process and analyze the output signals of the sensors. Current biomagnetometers utilize a sensor and sensor channel including a pickup coil which produces an electrical current output when a magnetic field penetrates the pickup coil. The electrical current, which is typically very small in magnitude, is detected by a Superconducting QUantum Interference Device, also known by the acronym SQUID. The pickup coil and SQUID normally operate in a superconducting state at reduced temperature. The output signal of the SQUID is provided to ambient-temperature electronics that process and filter the output signal, and thereafter the processed signal is analyzed to determine its relation to the operation of the human body.
Spurious effects from the detection of other magnetic fields than those produced by the brain can be removed by appropriate electronic signal filters. However, the ability of filters to remove all of the extraneous effects is limited. To further improve the signal-to-noise ratio of the system, the subject and pickup coil can be located in a magnetically shielded room.
Over the past 10 years, an important development in the field of biomagnetometry has been an increase in the number of sensors and sensor channels that are available on commercial units. That number has increased from 1 to 7, then to 14 and currently to as many as 37 sensors and sensor channels in a single biomagnetometer. The increase in the numbers of sensors is a highly desirable trend, because the ability to relate the magnetic signals measured by the sensors back to the functioning of the living organism can by improved by the analysis of large arrays of sensor signals, as discussed in U.S. Pat. No. 4,977,896.
As the number of sensors and sensor channels increases, the cost of the biomagnetometer increases accordingly. Although economies of scale and various manufacturing improvements have had some effect on controlling the increase in system costs, in general the larger biomagnetometer systems are much more expensive than the smaller systems. It is likely that future systems with 100 or more sensors and sensor channels will be even more expensive. The development of the field of biomagnetometry and the subsequent availability of this new tool to the general population may be inhibited by the expected large increases in system costs.
There is an ongoing need for an approach to the construction of very large sensor arrays without proportional increases in costs. While improvements in design and manufacturing techniques are helpful, they are not sufficient to reduce the projected systems costs to the extent desired so that large-array biomagnetometers will be priced for widespread use. The present invention fulfills this need, and further provides related advantages.